在python中从数独难题中提取网格

Question

I am currently learning OpenCV in python, and I am trying to draw over the contour of the grid on this image to extract the sudoku puzzle from it

This is the code I wrote for this specific problem:

CONST_IMAGE_PATH = "sudoku-original.jpg"
CONST_COEFF = 0.02
def main():
   originalImage = cv2.imread(CONST_IMAGE_PATH)
   img = cv2.imread(CONST_IMAGE_PATH,0)
   img = cv2.medianBlur(img,5)
   img = cv2.adaptiveThreshold(img , 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY,11,2)
   img = cv2.bitwise_not(img,img)
   print "thresholding the image"
   cv2.imshow("Thresholded", img)
   kernel = np.empty((3,3),'uint8')
   kernel[0][0] = 0
   kernel[0][1] = 1
   kernel[0][2] = 0
   kernel[1][0] = 1
   kernel[1][1] = 1
   kernel[1][2] = 1
   kernel[2][0] = 0
   kernel[2][1] = 1
   kernel[2][2] = 0
   dilated = cv2.dilate(img,kernel)
   cv2.imshow("Dilated", dilated)
   print "detecting the grid"
   (contours, _) = cv2.findContours(img.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
   contours = sorted(contours, key = cv2.contourArea , reverse = True)
   screenCnt = None


   for contour in contours: 
        perimeter = cv2.arcLength(contour,True)
        approx = cv2.approxPolyDP(contour, CONST_COEFF*perimeter , True)
        if len(approx) == 4: 
            if perimeter > maxPerimeter: 
                maxPerimeter = perimeter
                screenCnt = approx


   cv2.drawContours(originalImage , [screenCnt], -1, (0,255,0), 3)
   cv2.imshow("SudokuPuzzle", originalImage)
   cv2.waitKey(0)

However what happens is instead of drawing over the whole grid it just draws over the lower right box.

Why is this happening and what can I change in my code to draw over the whole grid ?

Answer 1

The one thing that was wrong, apparently:

(contours, _) = cv2.findContours(dilated.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

Should be dilated.copy() instead of img.copy() . At first I thought that it wasn't a big deal and the biggest contour should still be the grids edge, but testing shows that if you don't dilate the image then the biggest contour is this thing

And this is why it was ignored in the if len(approx) == 4 clause.